Avoid materializing slots only referenced by Kudu conjuncts

Right now, when SlotRefs are serialized to Thrift, Impala makes sure the slots are materialized. For Kudu we only need to mark them as materialized to build the kudu predicates, meaning the slots might not be referenced in the projection at all. In this case we should avoid materializing the slots (and thus allocating memory).

I believe that when we move to the scan token API we can avoid marking the SlotRefs as materialized.